1. Field of the Invention
The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer formed thereon, comprising an aromatic polycarbonate resin having charge transporting properties. In addition, the present invention also relates to a method of producing the above-mentioned aromatic polycarbonate resin with charge transporting properties.
2. Discussion of Background
Recently organic photoconductors are used in many copying machines and printers. These organic photoconductors have a layered structure comprising a charge generation layer (CGL) and a charge transport layer (CTL) which are successively overlaid on an electroconductive support. The charge transport layer (CTL) comprises a binder resin and a low-molecular-weight charge transport material (CTM) dissolved therein. The addition of such a low-molecular-weight gorge transport material (CTM) to the binder resin lowers the intrinsic mechanical strength of the binder resin, so that the CTL film becomes fragile. Such lowering of the mechanical strength of the CTL causes the wearing of the photoconductor or forms scratches and cracks in the surface of the photoconductor.
Although some vinyl polymers such as polyvinyl anthracene, polyvinyl pyrene and poly-N-vinylcarbazole have been studied as high-molecular-weight photoconductive materials for forming a charge transport complex for use in the conventional organic photoconductor, such polymers are not satisfactory from the viewpoint of photosensitivity.
In addition, high-molecular-weight materials having charge transporting properties have been also studied to eliminate the shortcomings of the above-mentioned layered photoconductor. For instance, there are proposed an acrylic resin raving a triphenylamine structure as reported by M. Stokla et al., in xe2x80x9cJ. Polym. Sci., vol 21, 969 (1983)xe2x80x9d; a vinyl polymer having a hydrazone structure as described in xe2x80x9cJapan Hard Copy ""89 p. 67xe2x80x9d; and polycarbonate resins having a triarylamine structure as disclosed in U.S. Pat. Nos. 9,801,517, 4,806,443, 4,806,444, 4,937,165, 4,959,298, 5,030,532, 5,034,296, and 5,080,909, and Japanese Laid-Open Patent Applications Nos. 64-9964, 3-221522, 2-304456, 4-11627, 9-175337, 4-18371, 4-31404 and 4-133065. However, any materials have not yet been put to practical use.
According to the report of xe2x80x9cPhysical Review B46 6705 (1992)xe2x80x9d by M. A. Abkowitz et al., it is confirmed that the drift mobility of a high-molecular weight charge transport material is lower than that of a low-molecular weight material by one figure. This report is based on the comparison between the photoconductor comprising a low-molecular weight tetraarylbenzidine derivative dispersed in the photoconductive layer and the one comprising a high-molecular polycarbonate having a tetraarylbenzidine structure in its molecule. The reason for this has not been clarified, but it is suggested that the photoconductor employing the high-molecular weight charge transport material produces poor results in terms of the photosensitivity and the residual potential although the mechanical strength of the photoconductor is improved.
Conventionally known representative aromatic polycarbonate resins are obtained by allowing 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A) to react with a carbonate precursor material such as phosgene or diphenylcarbonate. Such polycarbonate resins made from bisphenol A are used in many fields because of their excellent characteristics, such as high transparency, high heat resistance, high dimensional accuracy, and high mechanical strength.
For example, this kind of polycarbonate resin is intensively studied as a binder resin for use in an organic photoconductor in the field of electrophotography. A variety of aromatic polycarbonate resins have been proposed as the binder resins for use in the charge transport layer of the layered photoconductor.
As previously mentioned, however, the mechanical strength of the aforementioned aromatic polycarbonate resin is decreased by the addition of the low-molecular-weight charge transport material in the charge transport layer of the layered electrophotographic photoconductor.
It is therefore a first object of the present invention to provide an electrophotographic photoconductor free from the conventional shortcomings, which can show high sensitivity and high durability.
The above-mentioned first object of the present invention cap be achieved by an electrophotographic photoconductor comprising an electroconductive support, and a photoconductive layer formed thereon, comprising as an effective component an aromatic polycarbonate resin which is prepared by solution polymerization of a diphenol compound having a triarylamine structure with a dial compound of formula (I) 
wherein R1 and R2 are each independently an alkyl group which may have a substituent, an aryl group which may have a substituent, or a halogen atom: a and b are each independently an integer of 0 to 4: and Y is xe2x80x94COOxe2x80x94 or 
in which Z is a bivalent aliphatic group which may have a substituent or an, arylene group which may have a substituent.
A second object of the present invention is to provide a method or producing an aromatic polycarbonate resin that is remarkably useful as a high-molecular-weight charge transport material for use in an organic electrophotographic photoconductor.
The interfacial polymerization is widely used in the industrial production of aromatic polycarbonate resins. However, the interfacial polymerization has the drawbacks that impurities, that is, various electrolytes such as a salt and an alkali must be completely removed from the obtained polymer solution. Therefore, the industrial production by the interfacial polymerization needs many complicated techniques. In particular, some industrial skills of a high order are required to obtain the aromatic polycarbonate resin as a high-molecular-weight charge transport material with high purity.
The method of producing the aromatic polycarbonate by solution polymerization using a chloroformate is proposed in Japanese Laid-Open Patent Applications 8-269183, 9-151248 and 9-22135. However, the synthesis of chloroformate restricts the diversity of reaction, and in addition, it is difficult to obtain, a polymer with a high molecular weight by the above-mentioned solution polymerization method.
Furthermore, a diphenol compound having an ester linkage cannot be turned into a desired polymer by the interfacial polymerizaton because the diphenol compound undergoes the hydrolysis in the course of interfacial polymerization.
A third object is therefore to provide a method of producing an aromatic polycarbonate resin with a high molecular weight with no difficulty.
A fourth object of the present invention is to provide a method of producing an aromatic polycarbonate resin with charge transporting properties by carrying out the solution polymerization in a uniform liquid phase around at room temperature using a simple apparatus.
The second to fourth objects of the present invention can be achieved by a method of producing an aromatic polycarbonate resin, comprising the step of carrying out solution polymerization of a diphenol compound having a triarylamine structure with a diol compound of formula (I): 
wherein R1 and R2 are each independently an alkyl group which may have a substituent, an aryl group which may have a substituent or a halogen atom; a and b are each independently an integer of 0 to 4; and Y is xe2x80x94COOxe2x80x94 or 
in which Z is a bivalent aliphatic group which may have a substituent or an arylene group which may have a substituent.